Sustaining signals of spaced-apart positive and negative pulses for maintaining the glow in matrix gas display devices

ABSTRACT

The disclosure is of an information display system including a display panel which comprises a plurality of gas-filled cells, each of which is operated by a pair of leads which, when properly energized, causes the gas in the cell between them to glow. After a cell is energized and the energizing signals have been removed from the leads, a sustaining signal is applied which maintains the glow. The sustaining signal is made up of spaced-apart positive and negative waves which are produced by two separate transistor and diode pairs operating a series L-C circuit made up of an inductor and panel cells. The spacing between and the two waves is controlled by the spacing between input pulses which energize the two transistor and diode pairs.

IJnited tates atent Inventor George E. lllolz North Plainiield, NJ. Appl. No. 888,971 Filed Dec. 29, 1969 Patented Dec. 7, 1971 Assignee Burroughs Corporation Detroit, Mich.

Continuation of application Ser. No. 668,550, now abandoned and a continuation of 824,725, Apr. 25, 1969, now abandoned. This application Dec. 29, 1969, Ser. No. 888,971

SUSTAINING SIGNALS OF SPACED-APART POSITIVE AND NEGATIVE PULSES FOR MAINTAINING THE GLOW IN MATRIX GAS [56] References Cited UNITED STATES PATENTS 3,284,666 11/1966 l-lajicek 315/174 3,343,128 9/1967 Rogers 3l5/169X 3,379,831 4/1968 Hashimoto 315/169 X Primary Examiner- Roy Lake Assistant Examiner-E. R. La Roche A11orneys- Kenneth L. Miller and Robert A. Green ABSTRACT: The disclosure is of an information display system including a display panel which comprises a plurality of gas-filled cells, each of which is operated by a pair of leads which, when properly energized, causes the gas in the cell between them to glow. After a cell is energized and the energizing signals have been removed from the leads, a sustaining DISPLAY DEVICES signal is applied which maintains the glow. The sustaining 21 Claims, 41 Drawing Figs. signal is made up of spaced-apart positive and negative waves which are produced by two separate transistor and diode pairs "5}gm;'5 5 5';51%;;'g' g g g ggg operating a series L-C circuit made up of an inductor and I a Cl flosbzllzg panel cells. The spacing between and "the two waves is con- 313/109 5 trolled by the spacing between input pullses which energize the 193, 201, 217, 220; 315/160, 161, 167, 169, 171, and (Ode 280- i 1 2% w //0 T 7 l 1 1 1 o iiii s I I m J20 LOGIC V cmcuns flea f COLUMN 1 \1/ J \J M DRIVE w W L i6 lsrl INPUT 2Z9 INPUT 192 SUSTAINING SIGNALS F SPACED-APART POSITIVE AND NEGATIVE PULSES FOR MAINTAINING THE GLOW IN MATRIX GAS DISPLAY DEVICES This application is a continuation of application Ser. No. 668,550, now abandoned, and a continuation of copending application Ser. No. 824,725, filed Apr. 25, 1969, now abandoned.

BACKGROUND OF THE INVENTION Many types of display devices are known in the prior art,

and each has its own requirements in the way of driving circuitry for controlling the operation thereof. A relatively new display device uses a gas cell which comprises an insulating chamber filled with an ionizable gas and carrying two electrodes outside the chamber for applying electrical potentials across the gas. When a suitable pulse is applied, the gas glows, and, due to the formation of electrical wall charges, the glow can be sustained by the continuous or intermittent application of a sustaining potential. Thus, it can be seen that this new device has unique requirements for its operation which are not satisfied by known electronic circuits.

SUMMARY OF THE INVENTION DESCRIPTION OF THE DRAWING In the drawing:

FIG. I is a schematic representation of a system embodying the invention and a display device used therewith;

FIG. 2 is a sectional elevational view of the display of FIG.

FIG. 3 is a representation of a portion of a sustaining signal generated by the circuit of the invention; and

FIG. 4 is a modification of the circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawing, a system embodying the invention is adapted for operating a display panel which includes a plurality of distinct display cells 30 of the type which can be turned on by a first signal and held on by a smaller sustaining signal. In one form (FIG. 2), the display panel is made up ofa central apertured plate 40 having an aperture 50 for each cell and transparent top and bottom plates 60 and 70, respectively, which provide end walls for the cells. Each cell contains an ionizable gas such as neon, nitrogen, argon, etc., with a mixture of 93 percent neon and 7 percent nitrogen at an operating pressure of a few hundred mm. Hg being particularly suitable for the type of operation described above.

In the display panel 20, the cells 30 are arranged in rows and columns, and any number of cells may be provided, as is well known in the art. For convenience, three rows and three columns are shown. Each row of cells is provided with a single row lead or electrode 80 secured to the top surface of the top plate 60, and a single column lead or electrode 90 secured to the bottom surface of the bottom plate 70. The row and column electrodes are preferably transparent and are fon'ned of thin films of gold or the like. Electrodes 80 and 90 are capacitively coupled to their cells, and the cells comprise the capacitors in an LC circuit to be described.

The system of the invention includes circuit elements for converting input electronic information signals into signals which, when applied to the display panel, cause the desired symbol to be displayed. Such circuits are well known and inelude logic circuits, decoding circuits, and the like represented by block 100, and usually row line drive circuits I10 and column drive circuits 120 which apply energizing information signals to the row lines and the column lines to energize specific cells 30. Thus, in a typical case, a group of input signals applied to circuit I00 and representing a numeral, letter, or a message, produces a combination of pulses on the row lines 80 and column lines 90 such that selected cells 30 are energized and glow and display the information represented by the original input signal. This light output, which represents the storage or entry of information, can be sustained after the input signals to the row and column lines or electrodes have been terminated by other circuit elements shown and described below.

The sustaining circuit portion 128 of system I0 includes a first NPN-power switching transistor 130 and a second NPN- power switching transistor I40 represented in the usual fashion. The circuit also includes a first source of input signals which are rectangular pulses coupled to the primary winding of an input transformer 170. The secondary winding of transformer 170 has one end connected to the base of NPN-transistor 130, and its other end is coupled to the emitter electrode of transistor 130 which is also connected to the negative supply terminal of a power supply I90. The collector electrode of transistor 130 is connected through a highfrequency diode 200 oriented as shown to a bus 210.

Thesustaining circuit 128 also includes a second source 220 of input signals, which are rectangular pulses, coupled to the primary winding 230 of a second input transformer 240. The secondary winding 250 of transformer 240 has one end connected to the input or base electrode of NPN-transistor 140 and its other end connected to the emitter of transistor 140. The collector of transistor 140 is connected to the positive supply terminal of power supply 190, and the emitter is connected through a high-frequency diode 260 oriented as shown, opposite to diode 200, to bus 210. The bus 210 is connected through a resonating inductor 270 and through separate DC isolation capacitors 280 to each of the row electrodes 80 of the display panel 20. The common or ground terminal of power supply is coupled to bus 191 which is connected through DC isolation capacitors 310 to each of the column electrodes 90 of the panel.

Inductor 270 and the capacitors constituted by the display cells 30 comprise a series LC circuit which is driven by the sustaining circuit 128 in a manner to be described.

In operation of the invention, input information signals are applied and processed through logic circuitry 100, 110 and 120, and energizing pulses are applied to selected cells 30 to cause them to glow and provide a pattern of light which represents the information contained in the input signals. The input signals can be removed, and the pattern of glowing cells can be maintained by means of the sustaining circuit 128 which produces an alternating signal of smaller amplitude than the original energizing pulses. Fired cells generate wall charges which are utilized by the sustaining circuit to maintain glow.

In operation of the sustaining circuit 128, input pulses are applied alternately from input sources 150 and 220 to the transistors I30 and 140, respectively. Source 150 supplies an input triggering pulse through pulse transformer 170 to the base of transistor 130. This pulse has sufficient to turn on the transistor in the avalanche mode, and its length is less than one-half cycle of the resonant frequency of the series LC circuit so that it will not interfere with the subsequent turnoff of the transistor. With transistor I30 turned on, the power supply 190 applies a voltage to the LC circuit made up of inductor 270 and the turned-on cells 30, and in response thereto, cur rent builds up and decays in a sinusoidal fashion, as is characteristic of a series LC circuit. This action produces the first one-half cycle of an AC signal including sine wave voltage A which appears across the cells 30 and sine wave current C1 through the series circuit, with the voltage and current being 90 out of phase. At the end of the first one-half cycle, the current tends to reverse through the diode 200, but diode 200 cuts off, and the circuit remains quiescent with energy stored in the LC circuit, until the next input pulse is applied from input source 220.

After a selected time ll, input source 220 is energized and applies a pulse through the pulse transformer 240 to the base of transistor 140 and turns on this transistor in the avalanche mode. This permits current flow in the opposite direction through the series LC circuit and through diode 260 and transistor 140. This results in the second half-cycle of the AC signal including sine wave voltage B and sine wave current C2. As the second half-cycle is completed, the current flow through diode 260 attempts to reverse direction, and this diode cuts ofi'. Each half-cycle described has a frequency or period fixed by the resonant frequency of the LC components.

Alternate half-cycles of the AC signal are thus applied to panel in response to the application of pulses alternately and at selected intervals from input sources 150 and 220. The spacing between the alternate one-half cycles can be controlled by the spacing between the input pulses from sources 150 and 220, and these pulses are applied, and the alternate AC signals are repeated and applied to the ON cells at the frequency required to maintain the desired display visible. This required frequency to maintain glow is determined generally by the volume of cells 30, the gas in the cells, the gas pressure, and similar factors, as is well known to those skilled in the art. It might be noted that power supply 190 serves to compensate for loss of energy in the circuit during each cycle of operation.

In the system shown in FIG. 4, a circuit 300, known as a level change circuit, is inserted from power supply 190 and coupled through separate DC isolation capacitors 310 to each of the column lines 90 of the display panel 20. The circuit 300 includes two parallel paths, the first path including a switch 312 and a battery 320 in series, and the second path including a conductor in series with a switch 330. The circuit 300 is operated by closing one switch and then the other at a suitable rate and for short periods of time so that the battery can energize and turn on cells 30 that have been off for a relatively longtime so that some wall charge is always present, and the cells can be turned on relatively easily when the proper input information signals are applied to the panel and to these cells. This circuit 300 is operated at low frequency, and the cells are energized for such a short time that their glow is not visible.

The system of the invention has many advantages, among which is the fact that the turning on and off of the individual portions of the sustaining signal are readily controlled, and these operations can be performed at high speed by means of avalanche operation of the transistors and by means of high speed, high voltage diodes. in addition, operation of the circuit for generating the sustaining signal is independent of the display panel which is driven thereby. Another advantage is that the circuit requires no transfonners, which would be bulky and relatively complex to design.

What is claimed is:

l. A system for operating a display device of the type which comprises a cell containing an ionizable gas and carrying two external electrodes by which signals can be applied across the volume of gas in the cell, said system comprising means coupled to said two external electrodes for applying information signals to said cell and for firing the cell, and

a circuit coupled to said cell for applying thereto spacedapart positive and negative sustaining currents which can sustain the fired state of the cell after the information signals have been removed,

said circuit including means for generating said positive and negative currents separately and at selectively controllable intervals.

2. The system defined in claim 1 wherein said means for generating positive and negative currents includes (1) a first switching circuit which, when turned on, generates a positive voltage across said panel and turns ofi at about the maximum amplitude of said positive voltage, and (2) a second switching circuit which, when turned on, generates a negative voltage across said panel and turns off at about the maximum amplitude of said negative voltage.

3. The system defined in claim 2 and including a power supply coupled to each of said first and second switching circuits to compensate for loss. of energy therein during each cycle of operation.

4. The system defined in claim 2 and including a source of negative potential coupled to said first switching circuit and a source of positive potential coupled to said second switching circuit to compensate for loss of energy in each circuit during each cycle of operation.

5. The system defined in claim 1 wherein said means for generating positive and negative currents includes (l) a first switch and diode combination which, when turned on, generates a positive voltage across said panel and turns off at about the maximum amplitude of said positive voltage, and (2) a second switch and diode combination which, when turned on, generates a negative voltage across said panel and turns ofi at about the maximum amplitude of said negative voltage.

6. The system defined in claim 1 wherein said means for generating positive and negative currents includes (1) a first transistor and diode combination which, when turned on, generates a positive voltage across said panel and turns off at about the maximum amplitude of said positive voltage and 2) a second transistor and diode combination which, when turned on, generates a negative voltage across said panel and turns off at about the maximum amplitude of said negative voltage.

7. The system defined in claim 6 wherein said first and second combinations are connected across said cell and a resonating inductor in series therewith.

8. A system for operating a display device which consists of a panel of cells arranged in rows and columns, each containing an ionizable gas,

said panel having top and bottom surfaces,

a common row conductor in contact with each row of cells on said top surface and a common column conductor in contact with each column of cells on said bottom surface whereby input information signals applied to selected ones of said row and column conductors apply energizing potentials across selected cells and cause them to glow, said system comprising a circuit connected across all of said cells for applying thereto spaced-apart positive and negative sustaining currents which can sustain the fired state of the cell after the information signals have been removed,

said circuit being operable to generate said positive and negative currents separately and at selectively controllable times.

9. The system defined in claim 8 wherein said circuit for generating positive and negative currents includes (1) a first switching circuit which, when turned on, generates a positive voltage across said panel and turns 05 at about the maximum amplitude of said positive voltage, and (2) a second switching circuit which, when turned on, generates a negative voltage across said panel and turns off at about the maximum amplitude of said negative voltage.

10. The system defined in claim 9 and including a power supply coupled to each of said first and second switching circuits to compensate for loss of energy therein during each cycle of operation.

11. The system defined in claim 9 and including a source of negative potential coupled to said first switching circuit and a source of positive potential coupled to said second switching circuit to compensate for loss of energy in each circuit during each cycle of operation.

12. The circuit defined in claim 8 and including an auxiliary switching circuit connected across all of said cells for applying firing potential thereto for a period of time sufficient only to prime any cells which may not be in the fired state.

13. The system defined in claim 8 additionally including an auxiliary level change circuit,

means for selectively coupling said level change circuit across said row and column conductors of said cells for applying thereacross a firing potential, and

means for limiting the time duration during which said firing potential of said level change circuit is continuously applied to said conductors to less than that required to generate substantially visible glow in said cells whereby a quantity of wall charge is periodically generated in all said cells.

14. A system for operating a display device of the type which comprises a cell containing an ionizable gas and carrying two electrodes by which signals can be applied across the volume of gas in the cell, said cell and its electrodes being adapted to operate as a capacitor, said system comprising means coupled to said two electrodes for applying information signals across said cell and for firing the cell, and

a circuit coupled to said cellfor applying thereto spacedapart positive and negative sustaining currents which can sustain the fired state of the cell after the information signals have been removed,

said circuit including means for generating said positive and negative currents separately and at selectively controllable intervals.

15. The system defined in claim 14 wherein said circuit includes a power supply,

an inductor coupled to said cell and being resonant with said cell at a predetennined frequency to produce seriesresonant circuit operation,

a first combination of circuit elements coupled to said inductor and to said cell both for generating and applying across said cell a voltage of l polarity and for terminating said voltage at about its maximum amplitude and before the current through the cell can reverse its polarity, and

a second combination of circuit elements coupled to said inductor and to said cell both for generating and applying across said cell a voltage of the opposite polarity and for terminating said voltage at about its maximum amplitude and before the current through the cell can reverse its polarity.

16. The system defined in claim 15 wherein said combination of circuit elements comprises a first semiconductor switch in series with a first diode, and said second combination of circuit elements comprises a second semiconductor switch in series with a second diode,

a said first and second combinations of circuit elements being connected in parallel and said diodes being oriented to have one diode in one direction and the other in the opposite direction.

17 A system for operating a display device which consists of a panel of cells arranged in rows and columns, each containing an ionizable gas and each having the operating characteristics of a capacitor,

said panel having top and bottom surfaces,

a common row conductor in contact with each row of cells on said top surface and a common column conductor in contact with each column of cells on said bottom surface whereby input information signals applied to selected ones of said row and column conductors apply energizing potentials across selected cells and cause them to glow, said system comprising an inductor coupled to one of the electrodes associated with each of said cells,

d first combination of circuit elements coupled to said inductor and to said cells both for generating and applying across said cells a voltage of one polarity and for terminating said voltage at about its maximum amplitude and before the current through said cell can reverse its polarity, and

a second combination of circuit elements coupled to said inductor and to said cells both for generating and applying across said cells a voltage of the opposite polarity and for terminating said voltage at about its maximum amplitude and before the current through said. cell can reverse its polarity.

118. The system defined in claim 17 wherein said first combination of circuit elements comprises a first semiconductor switch in series with a first diode, and said second combination of circuit elements comprises a second semiconductor switch in series with a second diode,

said first and second combinations of circuit elements being connected in parallel and said diodes being oriented to have one diode in one direction and the other in the opposite direction 19. The system defined in claim 17 wherein the first combination of circuit elements applies a sine wave voltage to said cells and passes a sine wave of current in one direction through said selected cells, and

the second combination of circuit elements applies a sine wave voltage to said cells and passes a sine wave of current in the opposite direction through said selected cells.

29. The system defined in claim 17 wherein the first combination of circuit elements includes a first unidirectional switching device for controlling current flow through said inductor and said selected cells in one direction, and

the second combination of circuit elements includes a second unidirectional switching device oppositely poled for controlling trolling current flow through said inductor and said selected cells in the opposite: direction,

further including means for alternately energizing said first and second unidirectional switching devices at a variable rate to control the frequency of the sustaining current flow applied to said cells.

21. The system defined in claim i8 further including means for alternately energizing said first and second semiconductor switches in an avalanche mode to permit current flow through said selected cells and said inductor in alternately opposite directions,

said first diode operating to terminate said current flow a predetermined time after the first semiconductor switch has been energized,

and said second diode operating to terminate said current flow a predetermined period after said semiconductor switch has been energized. 

1. A system for operating a display device of the type which comprises a cell containing an ionizable gas and carrying two external electrodes by which signals can be applied across the volume of gas in the cell, said system comprising means coupled to said two external electrodes for applying information signals to said cell and for firing the cell, and a circuit coupled to said cell for applying thereto spaced-apart positive and negative sustaining currents which can sustain the fired state of the cell after the information signals have been removed, said circuit including means for generating said positive and negative currents separately and at selectively controllable intervals.
 2. The system defined in claim 1 wherein said means for generating positive and negative currents includes (1) a first switching circuit which, when turned on, generates a positive voltage across said panel and turns off at about the maximum amplitude of said positive voltage, and (2) a second switching circuit which, when turned on, generates a negative voltage across said panel and turns off at about the maximum amplitude of said negative voltage.
 3. The system defined in claim 2 and including a power supply coupled to each of said first and second switching circuits to compensate for loss of energy therein during each cycle of operation.
 4. The system defined in claim 2 and including a source of negative potential coupled to said first switching circuit and a source of positive potential coupled to said second switching circuit to compensate for loss of energy in each circuit during each cycle of operation.
 5. The system defined in claim 1 wherein said means for generating positive and negative currents includes (1) a first switch and diode combination which, when turned on, generates a positive voltage across said panel and turns off at about the maximum amplitude of said positive voltage, and (2) a second switch and diode combination which, when turned on, generates a negative voltage across said panel and turns off at about the maximum amplitude of said negative voltage.
 6. The system defined in claim 1 wherein said means for generating positive and negative currents includes (1) a first transistor and diode combination which, when turned on, generates a positive voltage across said panel and turns off at about the maximum amplitude of said positive voltage and (2) a second transistor and diode combination which, when turned on, generates a negative voltage across said panel and turns off at about the maximum amplitude of said negative voltage.
 7. The system defined in claim 6 wherein said first and second combinations are connected across said cell and a resonating inductor in series therewith.
 8. A system for operating a display device which consists of a panel of cells arranged in rows and columns, each containing an ionizable gas, said panel having top and bottom surfaces, a common row conductor in contact with each row of cells on said top surface and a common column conductor in contact with each column of cells on said bottom surface whereby input information signals applied to selected ones of said row and column conductors apply energizing potentials across selected cells and cause them to glow, saiD system comprising a circuit connected across all of said cells for applying thereto spaced-apart positive and negative sustaining currents which can sustain the fired state of the cell after the information signals have been removed, said circuit being operable to generate said positive and negative currents separately and at selectively controllable times.
 9. The system defined in claim 8 wherein said circuit for generating positive and negative currents includes (1) a first switching circuit which, when turned on, generates a positive voltage across said panel and turns off at about the maximum amplitude of said positive voltage, and (2) a second switching circuit which, when turned on, generates a negative voltage across said panel and turns off at about the maximum amplitude of said negative voltage.
 10. The system defined in claim 9 and including a power supply coupled to each of said first and second switching circuits to compensate for loss of energy therein during each cycle of operation.
 11. The system defined in claim 9 and including a source of negative potential coupled to said first switching circuit and a source of positive potential coupled to said second switching circuit to compensate for loss of energy in each circuit during each cycle of operation.
 12. The circuit defined in claim 8 and including an auxiliary switching circuit connected across all of said cells for applying firing potential thereto for a period of time sufficient only to prime any cells which may not be in the fired state.
 13. The system defined in claim 8 additionally including an auxiliary level change circuit, means for selectively coupling said level change circuit across said row and column conductors of said cells for applying thereacross a firing potential, and means for limiting the time duration during which said firing potential of said level change circuit is continuously applied to said conductors to less than that required to generate substantially visible glow in said cells whereby a quantity of wall charge is periodically generated in all said cells.
 14. A system for operating a display device of the type which comprises a cell containing an ionizable gas and carrying two electrodes by which signals can be applied across the volume of gas in the cell, said cell and its electrodes being adapted to operate as a capacitor, said system comprising means coupled to said two electrodes for applying information signals across said cell and for firing the cell, and a circuit coupled to said cell for applying thereto spaced-apart positive and negative sustaining currents which can sustain the fired state of the cell after the information signals have been removed, said circuit including means for generating said positive and negative currents separately and at selectively controllable intervals.
 15. The system defined in claim 14 wherein said circuit includes a power supply, an inductor coupled to said cell and being resonant with said cell at a predetermined frequency to produce series-resonant circuit operation, a first combination of circuit elements coupled to said inductor and to said cell both for generating and applying across said cell a voltage of 1 polarity and for terminating said voltage at about its maximum amplitude and before the current through the cell can reverse its polarity, and a second combination of circuit elements coupled to said inductor and to said cell both for generating and applying across said cell a voltage of the opposite polarity and for terminating said voltage at about its maximum amplitude and before the current through the cell can reverse its polarity.
 16. The system defined in claim 15 wherein said first combination of circuit elements comprises a first semiconductor switch in series with a first diode, and said second combination of circuit elements comprises a second semiconductor switch in series with a second diode, said first and second combinations of circuit elements being connected in parallel and said diodes being oriented to have one diode in one direction and the other in the opposite direction.
 17. A system for operating a display device which consists of a panel of cells arranged in rows and columns, each containing an ionizable gas and each having the operating characteristics of a capacitor, said panel having top and bottom surfaces, a common row conductor in contact with each row of cells on said top surface and a common column conductor in contact with each column of cells on said bottom surface whereby input information signals applied to selected ones of said row and column conductors apply energizing potentials across selected cells and cause them to glow, said system comprising an inductor coupled to one of the electrodes associated with each of said cells, d first combination of circuit elements coupled to said inductor and to said cells both for generating and applying across said cells a voltage of one polarity and for terminating said voltage at about its maximum amplitude and before the current through said cell can reverse its polarity, and a second combination of circuit elements coupled to said inductor and to said cells both for generating and applying across said cells a voltage of the opposite polarity and for terminating said voltage at about its maximum amplitude and before the current through said cell can reverse its polarity.
 18. The system defined in claim 17 wherein said first combination of circuit elements comprises a first semiconductor switch in series with a first diode, and said second combination of circuit elements comprises a second semiconductor switch in series with a second diode, said first and second combinations of circuit elements being connected in parallel and said diodes being oriented to have one diode in one direction and the other in the opposite direction.
 19. The system defined in claim 17 wherein the first combination of circuit elements applies a sine wave voltage to said cells and passes a sine wave of current in one direction through said selected cells, and the second combination of circuit elements applies a sine wave voltage to said cells and passes a sine wave of current in the opposite direction through said selected cells.
 20. The system defined in claim 17 wherein the first combination of circuit elements includes a first unidirectional switching device for controlling current flow through said inductor and said selected cells in one direction, and the second combination of circuit elements includes a second unidirectional switching device oppositely poled for controlling current flow through said inductor and said selected cells in the opposite direction, further including means for alternately energizing said first and second unidirectional switching devices at a variable rate to control the frequency of the sustaining current flow applied to said cells.
 21. The system defined in claim 18 further including means for alternately energizing said first and second semiconductor switches in an avalanche mode to permit current flow through said selected cells and said inductor in alternately opposite directions, said first diode operating to terminate said current flow a predetermined time after the first semiconductor switch has been energized, and said second diode operating to terminate said current flow a predetermined period after said semiconductor switch has been energized. 